Leigh syndrome (LS), also known as infantile subacute necrotizing encephalopathy, is the most frequent mitochondrial disorder in children. Recently, more than 80 genes have been associated with LS, which greatly complicates the diagnosis. In this article, we present clinical and molecular findings of 219 patients with LS and give the detailed description of three cases with rare findings in nuclear genes MORC2, NARS2 and VPS13D, demonstrating wide genetic heterogeneity of this mitochondrial disease. The most common cause of LS in Russian patients are pathogenic variants in the SURF1 gene (44.3% of patients). The most frequent pathogenic variant is c.845_846delCT (66.0% of mutant alleles; 128/192), which is also widespread in Eastern Europe. Five main LS genes, SURF1, SCO2, MT-ATP6, MT-ND5 and PDHA1, account for 70% of all LS cases in the Russian Federation. Using next generation sequencing (NGS) technique, we were able to detect pathogenic variants in other nuclear genes: NDUFV1, NDUFS2, NDUFS8, NDUFAF5, NDUFAF6, NDUFA10, SUCLG1, GFM2, COX10, PMPCB, NARS2, PDHB and SLC19A3, including two genes previously associated with Leigh-like phenotypes—MORC2 and VPS13D. We found 49 previously undescribed nucleotide variants, including two deep intronic variants which affect splicing.

The m.13513G>A (p.Asp393Asn) substitution in the MT-ND5 (Mitochondrially Encoded NADH/Ubiquinone Oxidoreductase Core Subunit 5) gene is a common pathogenic variant associated with primary mitochondrial disorders. It frequently causes Leigh syndrome and mitochondrial encephalomyopathy with lactate acidosis and stroke-like episodes (MELAS). In this study, we present clinical data, heteroplasmy levels in various tissues (blood, urine, and skin fibroblasts), and bioenergetic characteristics from a cohort of 20 unrelated patients carrying the m.13513G>A mutation, classified according to the following phenotypes: Leigh syndrome (n = 12), MELAS (n = 2), and Leber’s hereditary optic neuropathy (LHON, n = 6). We observed a significant correlation between high respiratory ratios and heteroplasmy levels in fibroblast cell lines of the patients. Furthermore, fibroblast cell lines with heteroplasmy levels exceeding 55% exhibited markedly reduced mitochondrial membrane potential. These findings contribute to a better understanding of the clinical and bioenergetic profiles of patients with m.13513G>A-variant-related phenotypes across different heteroplasmy levels, based on data from a single genetic center. Our data suggest that even a slight shift in heteroplasmy can improve cellular function and, consequently, the patients’ phenotype, providing a solid foundation for the development of future gene therapies for mtDNA diseases.

In 2023, the Russian Federation expanded its national newborn screening (NBS) program from 5 to 36 conditions, 29 of which are inherited metabolic diseases (IMDs). This study presents the first nationwide results and outcomes of the expanded NBS program. Between January 2023 and December 2024, dried blood spots from 2,466,615 newborns (98.53% of the birth cohort) were analyzed for IMDs using MS/MS. Screen-positive cases were referred to the national reference center for confirmatory testing, which included biochemical (MS/MS and GC-MS) and genetic analyses (NGS). A total of 41,728 neonates (1.69%) screened positive, of whom 37,733 underwent confirmatory testing. It resulted in 834 confirmed diagnoses of IMDs (1 in 2900 live births). Phenylketonuria was the most prevalent IMD (n = 538; 1 in 4600), followed by MCADD (n = 99; 1 in 25,000). Distinct regional and ethnic variations were observed, including a high prevalence of tyrosinemia type 1 in the Chechen Republic and MCADD in North Ossetia. The integration of NGS was essential for resolving complex cases, such as identifying heterozygous carriers and dual diagnoses. These findings underscore the program’s clinical utility, highlight unique epidemiological patterns, and identify challenges such as false positives and diagnostic complexities, which will guide future refinements.